There are many circumstances in which liquid process streams, used in industrial processes, become contaminated to an extent that it hinders their reuse. If this occurs, it is desirable to be able to clarify the streams in a quick and effective manner. However, since the equipment space is at a premium in many installations, clarifying techniques such as settling tanks or ponds are not always practical.
According to the present invention, it has been recognized that equipment like that used in the deinking of paper during recycling, and/or removing sticky contaminants, such as shown in U.S. Pat. Nos. 5,069,751 and 5,131,980 (the disclosures of which are hereby incorporated by reference herein) can successfully be employed to clarify liquid process streams, with only minor modifications. That is, according to the invention, it has been found that by using a compact gas sparged hydrocyclone on liquid streams having a solids consistency of less than about 0.5% (and more than 90% of the particles having a maximum dimension of less than 500 microns) it is possible to quickly and effectively clarify the liquid stream, e.g. to remove at least 60-70% of the suspended particles in the stream having a size between 10-200 microns, or between just above 0 and 150 microns.
The invention is particularly applicable to liquid process streams from pulp and paper mills, such as white water, pressate, or washer filtrate. White water is that water drained from a paper machine during the process of converting paper machine headbox feedstock into paper or market pulp. This water typically contains suspended solids such as conventional fillers (e.g. clays, calcium carbonate, etc.) used in the paper making operation as well as fibers which were not retained with the paper sheet formed. Washer filtrate is the liquid obtained as stock is thickened or washed. In a recycled fiber application, it can contain suspended solids such as inks and paper fillers and coating in addition to fibers not retained by the washer mat. The invention is particularly successful in clarifying these liquids in a highly efficient manner, while the equipment takes up little space.
The hydrocyclone used according to the invention is basically as shown in U.S. Pat. Nos. 5,069,751 and 5,131,980, and includes an inlet, rejects outlet, accepts outlet, a porous tube having a given internal radius and through which gas may be sparged, and a pedestal adjacent the accepts outlet and defining a clearance space. The porous tube may have a nominal diameter of about 2-15 inches. While in a typical deinking application the clearance is about 15% of the inner radius, when practicing the liquid clarifying method according to the present invention, the clearance is about 8-12% (e.g. about 10%) of the inner radius.
According to one aspect of the present invention there is provided a method of clarifying a liquid stream containing suspended solid particles, the stream having a solids consistency of less than about 0.5%, and more than 90% of the particles having a maximum dimension of less than 500 microns. The method comprises the steps of substantially sequentially and continuously: (a) introducing the liquid stream having a consistency of less than about 0.5% into a first end of a vortex; (b) sparging gas into the suspension in the vortex to cause particles to attach to gas bubbles and move back toward the first end of the vortex, while the majority of the liquid stream moves toward a second end of the vortex, opposite the first end; (c) removing the clarified stream from the second end of the vortex in an accepts stream, the accepts stream having a greatly reduced particles content compared to the liquid stream at the first end of the vortex; and (d) removing particles attached to gas bubbles from the first end of the vortex in a rejects stream.
The method is particularly useful where the liquid stream is a pulp and paper mill process stream, and it comprises the further step of (e) using the clarified liquid from step (d) in a pulp and paper mill process. Step (e) is preferably practiced immediately after step (d), without further treatment of the liquid in the stream.
The method may also comprise the further step of (e) adding surface charge modifying chemical to the liquid stream prior to, or during, the practiced of steps (a)-(c), to modify the surface charge of the particles in the liquid, e.g. by adding anionic, nonionic or cationic surfactant mixtures of the type currently used in dissolved air clarification or wastepaper flotation. The liquid may be a process liquid from a pulp and paper mill process stream, and has some cellulose fibers and/or filler therein; and step (e) may be practiced to alter the quantity of fiber and/or fillers in the accepts stream of step (c).
Step (a) may be practiced at conditions comparable to a feed pressure of about 15 psig and a flow rate of about 27 GPM, with the vortex having a diameter of about 2 inches, with step (b) practiced at a gas flow rate of about 4-5 scfm, and step (d) practiced to provide a rejects flow rate of about 1-2 GPM.
The method is preferably practiced using a hydrocyclone having an inlet, a rejects outlet, an accepts outlet, a porous tube of given radius through which the gas of step (b) is sparged, and a pedestal adjacent the accepts outlet and defining a clearance space; and there is preferably the further step of adjusting the clearance space so that it is in the range of about 8-12% of the porous tube given radius.
According to another aspect of the invention, a method of clarifying and utilizing a liquid stream selected from the group consisting essentially of white water, pressate, and washer filtrate from a pulp and paper mill, using a hydrocyclone having an inlet, rejects outlet, accepts outlet, a porous tube having a given internal radius and through which gas may be sparged, and a pedestal adjacent the accepts outlet and defining a clearance space, is provided. The method comprises the steps of: (a) adjusting the clearance space so that it is in the range of about 8-12% (e.g. 10%) of the porous tube given radius; then substantially continuously (b) introducing the liquid stream into the inlet; (c) introducing gas through the gas permeable tube; (d) withdrawing rejects from the rejects outlet, and accepts from the accepts outlet; and (e) reusing the accepts directly in the pulp and paper mill as process stream liquid.
It is the primary object of the present invention to provide a method for efficiently clarifying liquid process streams without taking up a substantial amount of space for the clarifying process. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.